Pressurized water nuclear reactors comprise a core formed from assemblies of prismatic shape arranged vertically and resting on a support plate, inside the vessel of the nuclear reactor.
During operation of the nuclear reactor, it is necessary periodically to perform flux measurements right inside the core. For this purpose fission detectors of very small dimensions are used which are displaced by remote control with the aid of teleflex cables inside tubes, called thimbles which are closed at one of their ends. The thimbles are introduced according to a predetermined distribution into certain assemblies of the core, after passing inside an instrumentation guidance duct. The instrumentation guidance duct comprises a guidance tube connecting a measurement room to the bottom of the vessel of the reactor, at the level of a penetration sleeve for the bottom and a vertical channel passing through the lower internals of the reactor, in alignment with the vertical tube-guide of the fuel assembly into which the thimble is introduced.
By displacement of the flux detectors inside the thimbles introduced into the assemblies, flux measurements may be carried out over the entire height of the core.
The thimbles must be able to be extracted from the assemblies of the core, for example in order to facilitate reloading of the core of the reactor or even of the set of corresponding instrumentation guidance ducts. To this end, traction is exerted on the end of the thimbles, from the measurement room arranged laterally with respect to the vessel well of the reactor.
When the thimbles are installed in their respective guidance ducts and in the tube guide of the corresponding assemblies, the closed end part of the thimble, generally consisting of a bullet-shaped end plug facilitating the guidance and the displacement of the thimble, is placed at a reference level in the upper part of the tube guide of the assembly.
The position of the thimble in the axial direction of the guidance duct is therefore fixed once and for all, and the same zones of the wall of the thimble are always placed facing the same zones of the guidance duct.
Due to the circulation of the cooling water of the reactor inside the guidance ducts, and in certain zones, for example before the inlet into the tube guide of the fuel assembly where the thimble is directly exposed to the water filling the vessel of the reactor, vibrations of the thimble are produced inside the guidance duct, due to the fact that the inside diameter of the guidance duct is substantially greater than the outer diameter of the thimble so as to allow the thimble to be extracted and installed in the guidance duct.
In certain zones of the guidance duct and in particular in the upper part of the vertical channel inside the internals, friction is produced generating wear between the wall of the thimble and the inner surface of the guidance duct.
The wear of the wall of the thimble in the zones subjected to friction may lead to deterioration and even to breakage of the thimble, due to the fact that the wear is always produced in the same zones of the wall of the thimble and during long periods of time corresponding to the duration of use of the nuclear reactor.
In the event of a deterioration of the thimble showing up as a loss in sealing, contaminated cooling liquid is likely to enter the measurement room via the inside of the thimble.
There have been proposed devices for holding the thimbles transversely inside certain parts of the guidance duct, allowing vibration, friction and wear to be limited.
However, such devices may be difficult to install in the guidance duct and may necessitate the use attached components which are likely to become detached by breaking and to become loose parts in the primary circuit of the reactor.
To date no method was known enabling the wear of the thimbles to be limited without using a transverse holding device.
The object of the invention is therefore to propose a method for using a measurement duct or thimble, closed at one of its ends, for a nuclear reactor cooled by pressurized water, so as to extend the service life of the thimble ensuring guidance of a probe measuring neutron flux right up as far as the inside a fuel assembly of the core of the reactor, the thimble being introduced into a guidance duct consisting of a guidance tube Joining a measurement room to the vessel the nuclear reactor in which the core is placed, a vertical channel passing through the lower internals of the reactor, beneath the core in line with a vertical tube guide of the assembly and the tube guide of the fuel assembly, so that the closed end of the thimble penetrates inside the tube guide of the assembly, as far as a predetermined level, this method making it possible to avoid excessive wear of the wall of the thimble in exposed zones, during extended use of the nuclear reactor.
With this objective, the position of the thimble is modified along the axis of the guidance duct, between two successive periods of continuous use of the thimble so as to ensure the guidance of the measurement probe in the operating nuclear reactor, so as to modify the position of at least one heavy wear zone of the wall of the thimble with respect to at least one zone of the guidance duct.
The invention also relates to a device for adjusting the position of a thimble in the axial direction of a guidance duct allowing the implementation of the method according to the invention.